Thursday, December 8, 2011

763. Pupillary light refex - A paralysed Shih Tzu

Yesterday, Dec 8, 2011, I was consulted about a Shih Tzu that was "paralysed". The sister had phoned me on Dec 7 for a 3rd opinion and I had asked her to send the dog in for examination as it is difficult to know what is going on by phone diagnosis.

The dog was treated by Dr Vanessa on Dec 7 and so I asked her where was the area of pain. I had seen the dog lying sideways in the crate. My assistant Min said Dr Vanessa had treated the dog. She came in around 11.30 am. I was about to talk to Dr Vanessa when the brother came in. This was the best time as it is always good for the owner to be involved in the examination.

AREA OF SPINAL PAIN
The dog had been treated by Vets 1 and 2 with steroids and so the area of spinal pain was not specifically known to Dr Vanessa when she received the dog. "Which area of the spine is painful?" I asked her since she had given a steroid injection and any pain would be masked.

The dog was able to lift his head but was unable to stand on four legs and that was definitely injury to the central nervous system. "Slight pain is further to the back area," Dr Vanessa said. It would be difficult to diagnose pain in the spinal area when the dog had been treated with pain-killers by Vets 1 and 2. Their medical records were brief. Acupuncture was done by Vet 2. But both of them had not done any X-ray. Later in the evening, the sister who had taken the dog to the vet spoke to me and said that she was given the choice of "X-ray" and/or "MRI."

There seemed to be some miscommunication here. Since I did not speak to Vets 1 and 2, I would presume that they had advised X-ray and the owners had opted out but the owner had said she was not a professional and should not be given a choice. It is extremely difficult nowadays. I usually advise X-ray in such traumatic cases and if the owner declines, I will record it down in my medical record. This is to avoid this type of "mis-communication".

For this report, I would focus on the pupillary light reflex done on the Shih Tzu in the presence of the brother. A bright light was shone directly on each eye to check the consensual and direct pupillary light reflex

DIRECT EYE REFLEX
Left eye - pupil does not constrict
Right eye - pupil constricts
So, what is the problem?

See notes from Wikipedia below.



Clinical significance

In addition to controlling the amount of light that enters the eye, the pupillary light reflex provides a useful diagnostic tool. It allows for testing the integrity of the sensory and motor functions of the eye.[1]

Under normal conditions, the pupils of both eyes respond identically to a light stimulus, regardless of which eye is being stimulated. Light entering one eye produces a constriction of the pupil of that eye, the direct response, as well as a constriction of the pupil of the unstimulated eye, the consensual response. Comparing these two responses in both eyes is helpful in locating a lesion.[1][5]

For example, a direct response in the right pupil without a consensual response in the left pupil suggests a problem with the motor connection to the left pupil (perhaps as a result of damage to the oculomotor nerve or Edinger-Westphal nucleus of the brain stem). Lack of response to light stimulation of the right eye if both eyes respond normally to stimulation of the left eye indicates damage to the sensory input from the right eye (perhaps to the right retina or optic nerve).[1]

Emergency room physicians routinely assess the pupillary reflex because it is useful for gauging brain stem function. Normally, pupils react (i.e. constrict) equally. Lack of the pupillary reflex or an abnormal pupillary reflex can be caused by optic nerve damage, oculomotor nerve damage, brain stem death and depressant drugs, such as barbiturates.

Normally, both pupils should constrict with light shone into either eye alone. On testing each reflex for each eye, several patterns are possible.[6]

Optic nerve damage on one side: (Example in parens.: Left optic nerve lesion)
The ipsilateral direct reflex is lost (Example: when the left eye is stimulated, neither pupil constricts, as no signals reach the brain from the left eye due to its damaged optic nerve)
The ipsilateral consensual reflex is INTACT (because light shone into the right eye can signal to the brain, causing constriction of both pupils via the normal oculomotor nerves)
The contralateral direct reflex is intact (because light shone into the right eye can signal to the brain, causing constriction of both pupils via the normal oculomotor nerves)
The contralateral consensual reflex is lost (because light shone into the eye on the damaged side cannot signal to the brain; therefore, despite the right eye's motor pathway (oculomotor nerve) being intact, no signals from the left eye are able to stimulate it due to the damage to the sensory pathway (optic nerve) of the left eye)

Oculomotor nerve damage on one side: (Example in parens: Left oculomotor lesion)
The ipsilateral direct reflex is lost (Example: when the left eye is stimulated, only the right pupil constricts)
The ipsilateral consensual reflex is lost (Example: when the right eye is stimulated, only the right pupil constricts)
The contralateral direct reflex is intact (because light shone into both eyes can still signal to the brain, and the pupil on the undamaged side will still be able to constrict via its normal oculomotor nerve)
The contralateral consensual reflex is intact (because light shone into the left eye can still signal to the brain via the normal optic nerve, causing attempted constriction of both pupils; the contralateral pupil constricts via its normal oculomotor nerve, but the ipsilateral pupil is unable to constrict due to its damaged oculomotor nerve)

I will write more later

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